Add namespace support to the proxy package

[LucasRoesler]

The proxy package needs to clearly support selecting functions by namespace in order to support

openfaas/faas-netes#511

[alexellis]

@LucasRoesler is this complete now or still pending for the proxy?

[LucasRoesler]

still pending, i think

[LucasRoesler]

@alexellis i am not sure a change is actually needed? I think this was opened originally to make sure we didn't forget. But the implementation in faas-netes kind of shows that the current interface seems to be flexible enough. We can probably close it unless you think there is a specific feature that we haven't been able to implement in faas-netes yet because of the proxy implementation

[alexellis]

Sounds good to me. Thank you for the detailed reply.